In order to minimize the susceptibility of overhead electrical cable to aerodynamically-induced vibrations and other related phenomena, it is desirable to produce cable having an elliptical cross-section, with the major and minor axes thereof rotated along the length of the cable. Various designs of such elliptical cable are known in the art. One particular design of such elliptical cable is disclosed in U.S. Pat. No. 5,171,942 entitled "Oval Shaped Overhead Conductor and Method for Making Same", commonly assigned to the assignee of the present application, and hereby incorporated by reference herein.
Although the problem of damping of undesired vibration has been solved by the invention of the above-identified patent, the actual manufacture of such cable so as to optimize the use of standard stranding machines has remained unaddressed in the prior art.
Stranded cables are typically produced on conventional stranding machines such as those manufactured by Krupp GmbH, Essen, Germany. Such stranding machines include a series of wire guides which provide the capability of stranding a plurality of concentric wire layers simultaneously. Such machines are designed to strand round or circular cross-section wires of uniform diameters. For example, Krupp Model No. KVS 1+12+18 has two sets of wire guides which provide the capability of stranding an inner layer of twelve wires simultaneously with an outer layer of eighteen wires. Krupp Model No. KVS 1+12+18+24 has three sets of wire guides which provide the capability of stranding an inner layer of twelve wires, an intermediate layer of eighteen wires and an outer layer of twenty-four wires. And yet another Krupp machine, as modified, has four sets of wire guides which provide the capability of stranding an inner layer of twelve wires, a first intermediate layer of eighteen wires, a second intermediate layer of twenty-four wires and an outer layer of thirty wires.
The numbers of wires in the respective layers is a function of conventional cable design in which cable having a substantially circular cross-section is produced by stranding round wires of uniform diameter. As a consequence, the packing of wires of uniform diameter in a closely packed matrix results in each succeeding layer having a predetermined number of wires therein. Conventional stranding machines are thus designed to optimize such production requirements using wires of a uniform diameter.
Specifically, geometry dictates that, in a closely packed matrix of uniform diameter wires, each successive layer will have a predetermined number of wires therein, in the progression of 1, 6, 12, 18, 24, etc. Thus, the conventional stranding machines are designed to produce successive layers in accordance with the dictates of geometry.
Departure from the standard practice of producing cables of essentially circular cross-section by stranding round wires of uniform diameter alters the geometry of close-packed matrices for non-circular cross-section cables. For example, stranding a cable having an elliptical cross-section alters the packing of wires because the same size wires are not used throughout. As a result, in order to properly fill the interstices in the conductor matrix, a greater number of wires may be required for a particular layer than would otherwise be required for a circular cross-section conductor. This alters the ratio of wires from the standard 1:6:12:18:24:30 progression, as measured for each layer in the outwardly radial direction.
While the elliptical cable designs can be stranded using standard stranding machines, the mixture or progression of numbers of wires required for each layer frequently alters the numbers of wires in each layer and requires the use of a larger capacity machine than would normally be justified in order to strand the cable in a single pass through the machine. Alternatively, smaller capacity machines can be used, but require more than one pass. In either instance, the cost of manufacture is increased and the efficiency reduced. The present invention addresses this problem.